Solar Energy, Vol.216, 180-187, 2021
Influence of donor units on spiro[fluorene-9,9 '-xanthene]-based dopant-free hole transporting materials for perovskite solar cells
Three spiro[fluorene-9,9'-xanthene] (SFX)-based organic molecules with different donor units were synthesized and used as dopant-free hole-transport materials (HTMs) in perovskite solar cells (PSCs). The end-capping donor units on the SFX core are two methoxydiphenylamine-substituted diphenylamine (MDDPA), two and four methoxydiphenylamine-substituted carbazole (MDCZ) for SFX-2DPA, SFX-2CZ and SFX-4CZ, respectively. The structure-function relationship is systematically investigated as the impact of the different donor units upon the optical, electrochemical and photophysical properties of SFX-based HTMs. The PSCs based on SFX-2CZ attain the higher power conversion efficiency (PCE) of 15.83% than that based on SFX-2DPA (14.25%), due to higher molecular planarity of SFX-2CZ with enhanced hole transporting ability and the downshifted HOMO level. On other hands, SFX-4CZ has the lowest efficiency (12.19%) among three HTMs mainly due to its inferior film forming ability. The results provide not only the new molecular design of dopant-free organic HTMs based on SFX core, but also a promising strategy to develop efficient HTMs with both the molecular planarity and number of donor groups being considered.